Cessna 206 G Forced Landing at Saint-Etienne Bouthéon (F-GDAA)

Incident Overview and Immediate Circumstances On 18 June 2011 at 18:00 local time, a Cessna 206 G, registration F-GDAA, operated by a Société de travail aérien, departed from the Saint-Etienne Bouthéon aerodrome. The aircraft was carrying four parachutists for the pilot's ninth drop of the day. Between 4,000 and 5,000 feet QNH, the pilot heard a loud noise from the engine, felt vibrations, and observed oil on the windscreen. The pilot declared an emergency, applied the in-flight engine stop procedure, and executed a forced landing on the runway. During the landing roll, the engine stopped. ## Engine Examination and Findings On the ground, two perforations were found on the upper crankcase at cylinders 3 and 4. The engine, a Continental IO-520F, was removed and examined. The investigation determined that the engine stop was caused by the failure of the connecting rod for cylinder 4 at its attachment to the crankshaft. The connecting rod failed due to a fatigue cracking process, which likely initiated from machining marks detected in a bolt housing bore. ## Maintenance History and Inspection Methods The engine was manufactured and installed on the F-GDAA in 2004. Between February and April 2011, it underwent a general overhaul during which a magnetic particle inspection (magnaflux) was performed. This inspection did not reveal any fatigue cracks on the connecting rods. At the time of the incident, the engine had accumulated 1,330 hours of operation, including 63 hours and 29 cycles since the general overhaul. Magnetic particle inspection involves applying an indicator product to the part and subjecting it to a magnetic field to reveal surface defects and some shallow subsurface defects. ## Conclusion and Contributing Factors The investigation considered two possible scenarios. First, the crack detection inspection during the general overhaul may have failed to detect the cracks, either due to operator oversight or because the magnetic particle inspection method was not suitable for this type of defect. Second, the fatigue cracks may not have been present at the time of the inspection, appearing and developing to failure within the 63 hours following the overhaul. The examination of the connecting rod did not identify an initiating factor, such as an impact during reassembly. Without a response from the engine manufacturer to the BEA's questions, the investigation could not determine which scenario was more probable. The presence of machining marks was identified as a contributing factor to the fatigue failure.